Dedicated I/O Delivers Faster Communication
We are all aware of how important it is to choose the right combination of hardware and software to deliver optimal results in a manufacturing automation application in specific circumstances, even though we may not always apply it. Putting this piece of well-worn wisdom to the test, Metro Weighing and Automation (MWA) discovered that a new I/O based system with logic and communications perfor...
We are all aware of how important it is to choose the right combination of hardware and software to deliver optimal results in a manufacturing automation application in specific circumstances, even though we may not always apply it.
Putting this piece of well-worn wisdom to the test, Metro Weighing and Automation (MWA) discovered that a new I/O based system with logic and communications performed better than its existing PLCs as applied for manufacturing weighing and packaging systems.
MWA, based in Taylor, MI, opened for business in 1971 as a scale sales and service company. Nine years later, the company developed the first automatic furnace loading system, which used scales and vibratory feeders. A decade later, the company developed its first packaging system, with an industrial computer controlling various components of the system. Today, about 90% of MWA's business is with providers of fasteners used in the automotive industry.
From hoppers to conveyors
Though MWA produces five distinct packaging systems, each is a custom unit configured to meet a customer's particular needs, says Philip Harrison, an MWA executive in charge of the control hardware and software in those systems. Essentially, a packaging system consists of hoppers, feeders, scales, and conveyors, which must be carefully synchronized to ensure that packages aren't overfilled or underfilled and that they are moved into place under the hoppers quickly. The interrelationship of the packaging system's parts and the need to communicate with other systems in the plant makes packaging applications ripe for automation.
"Typically, you have 10 or 12 controls that govern the equipment that feeds product into the scale hopper," says Harrison. "You have to be able to control how fast the feeders run, and quickly adjust feed rates and cutoffs. Ninety percent of the control functions are performed through a touchscreen PC running IntelliPack software, which we developed. The remaining control—primarily the box movement—was handled through the PLC."
MWA's systems performed well, but Harrison was frustrated for years by communications and processing bottlenecks in his systems' PLCs that forced him to develop inelegant software workarounds. The problem, he says, is that PLCs in that application simply didn't support the necessary data structures or deterministic processing that the packaging applications demand. In addition, the proprietary PLC networking protocols used were too slow, aggravating the situation.
"The PLCs we were using had turnaround times in the 30-50 ms range. The PLC vendor's paradigm is to make control on the inside of the device quick, but if you have an outside process that has to get to those data, that device is a last-class citizen," says Harrison. Following standards to get the data would have been more effective, he suggests.
"Our software was both polling inputs in the background and setting outputs as it made decisions. But even with optimized communication through asynchronous calls and block reads and writes, the communication lines were clogged, and the software was unable to adjust feeder speeds deterministically. We had to design our software to actually pause for up to 300 ms in cases where the I/O queue got too large. During that time, the software would be blind to incoming scale data until it could make another I/O call. That's an unnerving thing, but there wasn't much else that could be done in that situation."
The problem prompted Harrison to seek other means of enabling control. Opto 22's Snap Ultimate I/O provided a speedy communications channel to control points on the packaging system, and has standard 10 Mbps and 100 Mbps Ethernet communications, which allows data exchange with enterprise databases without middleware; some control functionality embedded in the I/O module land permits a distributed I/O architecture.
"A PLC is, by its very nature, a single-thread device," says Anthony J. Dern, an Opto 22 engineer. "But the control side of our devices involves multitasking. So, we can do all the things that need to be done very quickly and deterministically."
Instead of using PLCs, MWA now mounts the Ultimate I/O bank and the Ultimate Brain controller on its packaging machines. An Ethernet crossover is used for communications between industrial PCs running MWA's Intellipack software and the I/O devices. MWA also uses Opto 22's Terminal-G70, an Ethernet-enabled, graphics-based operator interface with Opto 22 Snap Ethernet I/O and SNAP Ultimate I/O systems.
The difference in performance between these I/O systems and the previous PLCs is huge, says Harrison. MWA had been using two PLC models, one of which took 30 to 50 ms for an I/O call, while the other took 20 to 40 ms. In comparison, Opto 22's Ultimate Brain takes 0.5 to 1.5 ms to perform the same function. Additionally, standards, sample source, and available OPC drivers eliminated the need for proprietary protocols, Harrison says.
The I/O system facilitates physical maintenance of the packaging systems and simplifies the control panel. If a scale indicator fails, for example, technicians previously would have had to pry a circuit board off the unit, replace it, and close the case. Now it's a matter of popping a self-contained module in and out, according to Harrison.
MWA also simplified its system programming through use of Opto 22's ioControl, a graphical, flowchart-based programming tool for industrial automation, remote monitoring, and data acquisition applications. Opto 22's Dern says, "Within the graphic programming package, you define each task by building a flowchart of what needs to be accomplished. You can create several flowcharts for each individual task.... The instructions themselves are located inside each block of the flowchart."
Harrison says the faster I/O system enabled MWA to make the new version of Intellipack much more adaptable to dynamic conditions than was the original version. As a result, the systems are easier to preconfigure and more flexible for the users.
Intellipack Version 1, designed for PLCs, could not monitor conditions fast enough to make intelligent decisions, says Harrison.
"This forced us to combine decision-making with preset values that we had to set up on a utilities screen at the factory after running a mix of the customers parts," he says. "As a result, if the customer introduced a new part that wasn't similar to something we tested, we would have to go out into the field and make adjustments accordingly." The speed of communications via Opto 22 Ultimate I/O, however, allows the new version of Intellipack software to quickly "learn" a new part and adapt to it automatically.
"All we have to enter on the utilities screen is information about the feeders being used. Intellipack Version 2 constantly analyzes the parts as it is being run and makes very precise decisions to obtain optimal speed and accuracy," Harrison says. Snap Ultimate I/O and MWA's Intellipack software have given MWA the ability to deliver systems to its customers that require them to do little more than press the "start" button, says Harrison.
"To our knowledge we're the only ones that can deliver this kind of system," he says. "One customer bought three pack lines from us, essentially replacing their entire existing packaging operation with ours."
While the customer's old system should have been packing 15 cartons per minute, complex setup procedures and other problems resulted in producing less than half that number.
"But with our system, all MWA has to do is touch a part number, start the system, and they get 15 boxes per minute all day long."
Future developments include a module that will allow users to direct connection of the I/O system to scales.
Dan Sussman is a freelance writer based in Scottsdale, AZ.